1. Field of Invention
This invention relates generally to a sealing assembly for a rotary drive shaft that extends through the hull of a boat to couple an inboard marine engine to an outboard propeller, and more particularly to an assembly of this type adapted to accommodate a range of different shaft sizes.
2. Status of Prior Art
In a propeller driven boat provided with an inboard marine engine, the engine is coupled by a rotary drive shaft which extends through a tube in the stern of the boat to an outboard propeller. In order to prevent sea water from leaking into the boat through the stern tube, a sealing assembly is required for this purpose which permits the shaft to rotate, but blocks the passage of sea water into the boat.
Of prior art interest are the sealing assemblies disclosed in the Newton et al. U.S. Pat. Nos. 5,370,400 and 5,503,404. In each of these assemblies, a cylindrical bearing is provided through which the shaft extends. Engaging the shaft adjacent to one end of the bearing is a lip seal that serves to prevent sea water passing through the bearing from leaking into the boat.
Formed within the cylindrical body of the bearing is an annular reservoir within which the lip seal is positioned. The engine's cooling water is fed into the reservoir and discharged therefrom through a channel in the shaft back to the sea. This circulating flow of water acts to lubricate and cool the lip seal and thereby maintain it in working order. In the absence of such lubrication, the heat generated by the friction between the shaft and the seal would in time render the seal ineffective.
In the sealing assemblies disclosed in the Newton et al. patents, the cylindrical bearing is formed of an ultra-high molecular weight polymer (UHMW-polyethylene) possessing a very low coefficient of sliding friction. This cylindrical bearing for the drive shaft is coupled to the through tube in the stern of the boat by a flexible rubber hose that surrounds the shaft. Thus sea water entering the stern tube is confined within the hose and the bearing.
The Newton et al. sealing assemblies operate efficiently and are highly reliable. However, these assemblies are useable only with drive shafts for which they are expressly designed. Thus the UHMW bearing must be machined to have an internal diameter that will accommodate a given drive shaft size and no other. If therefore it becomes necessary to provide a sealing assembly for other drive shaft sizes, UHMW bearings must then be machined to accommodate each of these different sizes. And one must also provide various other components for the assembly that are appropriate to the shaft sizes.
Because of its extremely low friction characteristics, the use of UHMW material is desirable in a marine shaft bearing. But the nature of this material is such that when subjected to excessive compression, it becomes distorted and cold flows. In a sealing assembly of the Newton et al. type, the hose from the stern tube is joined to the UHMW bearing and is clamped thereto by metal bands. When these bands are tightened to avoid leakage, they may then subject the UHMW material to a compressive force sufficient to cause this material to cold flow. As a consequence, the UHMW bearing will bind on the shaft passing therethrough and thereby resist rotation of the shaft.
Another drawback of a UHMW bearing in a sealing assembly is that water lubrication is essential in order to ensure that the shaft rotates freely within the bearing. Should no lubricating water be present as a result of a blockage or a pinched line, the UHMW bearing will then immediately heat up and grab the shaft, the bearing material then softening. Should a catastrophic failure of the bearing cause the hose to tear free, water will then gush into the boat through the stern tube.
In sealing assemblies of the type disclosed in the Newton et al. patents, a lip seal of the appropriate size is pressfit with a 0.030 interference into one end of the UHMW bearing to ensure that no leakage occurs around the perimeter of the seal. Encircling this end of the bearing is a stainless steel band. Because of the tight press fit of the lip seal, it becomes very difficult to remove a worn seal and to replace it without in the process of doing so, damaging the bearing or nicking and scratching the shaft.
Yet another practical problem encountered with a sealing assembly of the Newton et al. type is that its cylindrical UHMW bearing which is coupled by a flexible base to the stern tube in the boat cannot be accommodated to stern tubes of different size. The reason for this limitation is that the external diameter of the bearing is machined to match the diameter of a given stern tube. In this way, one end of the coupling hose fits over the flow tube and the other end over the bearing. But if the boat has a stern tube of larger diameter than that of the bearing, a hose that fits onto this stern tube will be not fit snugly on the bearing.